mysql-test/r/derived.result:
  explain of hidden subselect changed according to review
mysql-test/r/subselect.result:
  explain of hidden subselect changed according to review
mysql-test/r/union.result:
  explain of hidden subselect changed according to review
sql/sql_class.h:
  we bo not need sign for now
sql/sql_lex.h:
  we do not need sign for now